This invention relates to a high-energy, low-voltage surge arrester for protecting electric circuit components against surges that are capable of rising at extremely high rates to thousands of amperes. The invention is particularly concerned with a surge arrester for use in low-voltage circuits rates at 25 volts or higher and for protection against surges having an energy content of 100's of watt-seconds or more.
There are now available high quality metal oxide varistor (MOV) disks that are sufficiently thin and sufficiently large in cross section as to provide effective high energy surge protection for low-voltage power circuits. For example, applicant's assignee has produced MOV disks of up to 3 inches in diameter and as thin as 0.03 inches. A typical disk for applicaton on a circuit rated at 120 volts normal (132 volts maximum) would be 3 inches in diameter and 0.05 inches thick. Such a disk exhibits a break-over voltage of about 350 volts crest at 10 KA on a wave of 8.times.20 .mu.Sec and is capable of absorbing 1 kJ of energy without damage.
The term "disk" as used herein is intended to comprehend within its meaning any structure of plate-like form irrespective of its peripheral configuration, whether circular, rectangular, or otherwise. The term "disk structure" as used herein is intended to comprehend one or more disks.
One application for an MOV disk of the type referred to hereinabove is in the military for protection of various installations against nuclear electromagnetic pulse (NEMP) surges. It is known that such pulses produce very fast rising current discharges requiring response times from a protective device on the order of 10 to 100 nanoseconds (nSec). In other applications such as the protection of circuits feeding sensitive equipment incorporating solid state devices and microprocessors, the response of the protective device to nanosecond phenomena may also be of importance. Examples of such equipment are medical equipment such as CAT scanners and MR imaging machines, computers, and microprocessor based process equipment. At the present time, there are many protective devices being marketed which are advertised as having nanosecond response, but these claims are often greatly exaggerated.